The present invention relates to a method of forming a through-hole and a through-hole forming machine, more precisely relates to a method of forming a through-hole in a circular wall of a cylindrical part of a work piece and a through-hole forming machine capable of performing the method.
Conventionally, through-holes are formed in work pieces by drill means, die-punch press means, electric spark means, etc. To form a through-hole in a circular wall of a cylindrical part of a work piece, e.g., pipe, the above described means have been used.
However, by using the drill means an the press means, burrs are formed along edges of through-holes, so they must be removed. Especially, in case of forming a thorough-hole from an outer face of the cylindrical part by the drill means or the press means, burrs are formed along an inner edge of the through-hole. If the work piece is small, it is difficult to remove burrs formed in the cylindrical part of the work piece. Further, in some work pieces, it is impossible to remove burrs.
Conventionally, in case of forming a through-hole in a relatively thick circular wall of a cylindrical part of a small work piece, the electric spark means has been used.
However, it takes a long time to form the through-hole by the electric spark means, so manufacturing efficiency must be lower. Further, machining cost must be increased.
In the mean time, a through-hole can be formed in a flat work piece by the press means as shown in FIGS. 20(a) and 20(b). The method is disclosed in Japanese Patent Gazette No. 5-42330 (paragraphs [0005], [0006], [0009] and [0019]).
In the method, circular grooves 174a, which correspond to an edge of a through-hole 170a to be formed, is previously formed in at least a bottom face of a flat work piece 107b, then a pierce punch 151a, which is arranged to correspond to the circular grooves 174a, is driven into the work piece 107b, so that the through-hole 170a can be bored. Note that, a symbol 178a stands for a scrap, which is a part of the work piece 107a separated by boring the through-hole 170a. 
However, in the method shown in FIGS. 20(a) and 20(b), the punch 151a is merely driven into a die 151b from an upper side. This method cannot be applied to form a through-hole in a circular wall of a cylindrical part of a small work piece.
Thus, workers have tried to bore a through-hole, by a punch, from the inside of the cylindrical part so as not to form burrs therein. However, a small punch, which can be inserted into the cylindrical part, is required, and a span of life of the punch must be short. Namely, there is no punches having such function.